The mechanisms by which intestinal epithelium communicates with the gut flora to maintain a healthy equilibrium capable of tolerating commensal bacteria while swiftly responding to specific pathogens is not well understood. Dynamic interactions between microbiota and the host modulate cellular proliferation, production of secretory cells, and gut-associated immune cells. Dysregulation of this interaction may contribute to the pathogenesis of inflammatory bowel disease. Additional evidence also supports a metabolic role for microbiota that ferment otherwise indigestible dietary nutrients, rendering them available for host absorption. The absence of these symbiotic flora, as seen in germ free animals, decreases caloric uptake from the diet and prevents obesity in animals exposed to high caloric density diets. Inoculation of germ free mice with normal gut microbiota restores their susceptibility to obesity. Host factors clearly modulate gut flora and the efficiency of nutrient uptake from the diet, as well. Gut Paneth cells produce antimicrobial peptides that (in addition to defending against gut pathogens) control the relative abundance of specific gut microbes. Additionally, the hormone leptin (which is produced by adipocytes in proportion to energy stores, and is thus reduced in starvation) may contribute to the regulation of gut flora. Leptin-deficient (ob/ob) mice display an altered gut microbial environment relative to controls, and the ob/ob flora promote more efficient nutrient uptake from the gut. To understand how leptin might modulate the gut and its associated flora, we examined the intestines of mice to determine the potential for direct leptin action via the leptin receptor (LepRb)- revealing LepRb expression in the Paneth cells of the distal ileum. We thus hypothesize that leptin acts directly via LepRb on Paneth cells to modulate interactions between the gut epithelium and microbiota and thus to control the relative balance of organisms in the gut flora. This role of leptin may be important not only for normal Paneth cell - gut flora interaction but also for metabolism as changes affecting overall efficiency of the bacterial community in the host can affect caloric extraction from dietary intake. The goal of this project is to understand roles for leptin/LepRb relative to dietary factors in the control of Paneth cell function and gut flora. We will thus utilize a variety of dietary and genetic manipulations in mice to determine the importance of leptin in Paneth cell function and in the balance of intestinal flora. We will also determine the role for Paneth cell LepRb in these measures and in metabolism by studying a mouse model in which LepRb is deleted from Paneth cells. This work will increase our understanding of the complex interactions between gut epithelium and microbiota which may contribute to pathogen defense, nutrient absorption and energy balance of the host. PUBLIC HEALTH RELEVANCE: Changes in gut microflora have been implicated in various human diseases such as enteritis/colitis, inflammatory bowel disease and obesity. The mechanisms by which intestinal epithelial cells communicate with and regulate luminal bacteria are poorly understood. We have recently identified receptors for the adiposity hormone leptin on Paneth cells specifically within the terminal ileum. Our primary aim is to determine the physiologic role of leptin signaling in ileal Paneth cells and test the hypothesis that leptin moderates gut microflora through Paneth cell secretion of antimicrobial peptides. These studies will aid in our understanding of ileal eplithlial cell regulation of microbiata and determine if alterations in this environment could contribute to the development of obesity, possibly through alterations in nutrient absorption.